A large number of methods has been proposed for the detection of gases. Particularly the detection technology using a small-sized indicator is a current focus of attention in the light of Its convenience in use and economic advantage. For example, there are known high-sensitivity gas indicators (Japanese Unexamined Patent Publication H8-122318 and H7-49342) each comprising a gas indicator body formed in a generally plate-like configuration and internally defining a passageway for admitting a specific gas to be detected and an indication layer adapted to react with said specific gas to undergo change in shade as affixed along said passageway to the inside wall of the body, said gas indicator body having a gate having a predetermined sectional area through which said specific gas may immediately come into contact with the indicator body and said indication layer being formed in the zone of said gate.
However, this prior art has the drawback that when a gas of low diffusibility is present only in a low concentration, its CT value (gas concentration×exposure time) can hardly be detected. This problem can be resolved by increasing the gas detection sensitivity. However, if the gas detection sensitivity is increased, the whole indicator undergoes a change in color instantly when exposed to a high concentration of gas so that the new problem occurs that its CT value can hardly be detected.
Thus, the development of a gas indicator applicable to both high and low gas concentrations was awaited but such an indicator remained to be developed and implemented yet.
It is, therefore, a primary object of the invention to provide a gas indicator applicable over a broad range of gas concentrations from a gas of low concentration to a gas of high concentration.